The invention relates to a seat belt buckle monitor switch indicating the closed state of a seat belt buckle in a motor vehicle and connected to a resistor network, wherein the switch has a switching finger supporting at least one first contact element which is able to make an electrical connection with at least one second contact element in dependence on the position of the switching finger.
Special switching devices for monitoring the closed state of seat belt buckles in motor vehicles are generally known. They are activated by a closed position of the seat belt buckle latch in the seat belt buckle. Thus, depending on whether the switch is opened or closed, a signal which corresponds to a safety belt which has not [or has] been put on by the occupant can be generated. A corresponding signal can be used for producing a warning signal for prevention of the motor vehicle being put into operation, etc.
To increase reliability, the prior art (FIG. 1) teaches that it is desirable to not trigger corresponding monitoring circuits directly by a switch, but rather to connect such a switch together with a voltage divider, consisting of two ohmic resistors, in such a way that one of the two resistors is bridged if the clearance between contacts is shorted. Depending on the application of a constant current or a constant voltage, the voltage then established along the series circuit (or the current flowing through it) can be evaluated. The information on switch closing appears within certain allowed tolerances, encoded into two permissible resistance values. In this way, a substantial independence of a closing signal from finite transfer resistors of the shorting switch is achieved.
Higher reliability requirements are obtained if such a switching device is not only used for alarm purposes, but is also operated as a sensor to influence the triggering threshold of another restraining device. For example this could be an air bag actuating device which is controlled in dependence on whether a vehicle occupant has put on the safety belt or not. For example, German Offenlegungsschrift 3,413,768 describes a corresponding occupant retaining system.
Prior art devices do not permit one to know whether or not the switch itself is defective, if a closing signal does not occur, or whether its electrical paths of connection to the said resistor network are defective.
An object of the instant invention is to provide a sensorily very reliable seat belt buckle monitor switch for indicating the closed state of a seat belt buckle in a motor vehicle.
This object is achieved in a cooperative circuit for monitoring the proper functioning of a switching component indicating the closing state of a seat belt buckle by having the switch include at least one first and second conductor bridge, each with a beginning terminal and an end terminal. A switching finger supporting the first contact element is electrically connected to the course of the first conductor bridge between its beginning terminal and end terminal. The second contact element is electrically connected to the course of the second conductor bridge between its beginning terminal and end terminal. The resistor network is connected to the beginning terminal of the first conductor bridge and to two separate lines connected to the beginning and end terminals of the second conductor bridge and has a further input.
It is also advantageous if the resistor network comprises only two electrically non-interconnected resistors which together with the second conductor bridge form a voltage divider. Additionally the switch can also have a third conductor bridge with beginning and end terminals and with a third contact element electrically connected to the course of the third conductor bridge between its beginning terminal and end terminal. Here the first contact element can establish an electrical connection with said third contact element in dependence on the position of the switching finger. Also a further resistor network is provided which is likewise connected to the beginning terminal of the first conductor bridge and via two corresponding lines to the beginning and end terminals of the third conductor bridge and has a further input.
It is also advantageous if the further resistor network comprises only two electrically not interconnected resistors, which together with the third conductor bridge form a voltage divider. At least one resistor network is also connected to the vehicle mass.
It is further advantageous if the two resistor networks are structurally combined into one unit and if the resistors are arranged either as thick-film or thin film resistors on an integral substrate.
In an advantageous way, the device according to the invention also allows all the leads to the switch to be monitored and an impermissible total resistance of the enquiry circuit in the closed state of the seat belt buckle to be assigned unambiguously to a defect of the switch, or to its activating elements, or to the test current path.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.